1. Field of the Invention
The present invention relates to lithographic printing.
2. Description of the Related Art
Lithographic printing is a process which utilizes a coated metal or paper plate containing an image (hydrophobic) area and a non-image (hydrophilic) area.
The image area will accept ink and the non-image area will accept water.
While some water will emulsify with the ink, no ink can be allowed to adhere to the non-image area. When it does, it is referred to as scumming and action must be taken to clean and coat the plate with gum to correct the problem.
A printing station on a typical lithographic press contains an inking unit, a dampening unit, a plate cylinder, a blanket cylinder, a nip roll and a gripper bar system to transport the sheet on a sheet fed press or a series of rollers to transport a web. During the printing cycle, the ink is transferred to the plate from the ink unit through a series of rollers and the water which contains other components is transferred from the dampening unit to the plate, the combination of ink and water are transferred to the blanket cylinder which is covered with a rubber or urethane blanket and then to the substrate.
On sheet fed presses, the printed sheets are conveyed to a delivery pile where the sheets are stacked one on top of the other. The ink is still wet at this point and must be prevented from transferring or offsetting from the printed surface to the bottom of the next sheet.
Conventional sheet fed presses incorporate spray units in the delivery section which spray the sheets with a film of powder to reduce or prevent the offsetting condition until the ink dries.
Another method of coping with the problem is to apply an aqueous coating to the printed surface and then dry it with infrared radiation.
Conventional high speed web presses use specially formulated heat set inks and incorporate very large drying system which are expensive to purchase, expensive to operate, and require highly sophisticated emission control equipment in order to comply with clean air standards.
With known lithographic printing processes, changes in printing press performance and printing results may be affected through variation of the ink formulations. Catalytic agents are introduced directly into the ink. Printing inks have been modified in this way for various specific requirements such as adhesion, abrasion resistance, ductility hardness, opacity, transparency, color, shading, gloss and density.
For example, U.S. Pat. No. 3,682,688 to Hughes et al., discloses adding a compound to lithographic printing ink to accelerate its drying. U.S. Pat. Nos. 4,659,848 and 4,705,568 both to Kay et al., add compounds to lithographic ink to promote its adhesion to plastic substrates.
U.S. Pat. No. 4,906,296 to Zweig discloses a fountain solution for transporting catalytic cross-linking agents to lithographic printing inks, infusing the inks with the catalytic cross-linking agents and making them reactive to ultraviolet radiation, infrared radiation or hot air. When this fountain solution is used, in conjunction with the described drying apparatus, the ink does not offset even when no spray powder is used. It is necessary however, to add from 10% to 20% isopropyl alcohol to the fountain solution mixture in order to keep the printing plate from scumming. This is undesirable since the isopropyl alcohol evaporates and mixes with other elements to form ozone in the atmosphere.